Non-aqueous electrolyte secondary batteries used as electric power sources for portable electronic equipments in recent years include a lithium-containing transition metal oxide in the positive electrode and a carbon material capable of absorbing and desorbing lithium in the negative electrode, thereby having high power and high energy density. The positive and negative electrodes include respective binders for binding active material particles together. As the binder in the negative electrode, used are polyvinylidene difluoride (PVDF) or styrene-butadiene rubber (SBR), for example.
To impart sufficient strength to the negative electrode, it is necessary to mix a large amount of binder with a negative electrode active material such as the above carbon material. If a large amount of binder is used, however, the surface of the carbon material is covered with the binder. This reduces the surface area of the carbon material contributable to charging and discharging reaction, and thus deteriorates the high-rate discharge characteristics and the low-temperature characteristics of the battery. To compensate this, the salt concentration in the non-aqueous electrolyte must be increased.
However, increase of the salt concentration will enhance the reactivity of the electrolyte when the battery is under a high temperature and overcharged. Therefore, the battery temperature tends to easily rise, and thus the safety may be impaired.
In addition, if most of the surface of the carbon material is so covered with the binder that the surface area of the carbon material contributable to the charging and discharging reaction reduces, the carbon material fails to absorb a sufficient amount of Li. As a result, metallic Li is deposited on the surface of the carbon material, and thus the safety of the battery may further be impaired.
Also, the high-rate discharge characteristics of the batteries are greatly influenced by the affinities between the non-aqueous electrolyte and the electrodes, which depend on the amount and kind of the binders. If the permeability of the non-aqueous electrolyte into one of the electrodes is too high, the distribution of the non-aqueous electrolyte inside the battery is nonuniform, and thus the high-rate discharge characteristics are impaired.
The present invention relates to a non-aqueous electrolyte secondary battery. The present invention also relates to a negative electrode for a non-aqueous electrolyte secondary battery.
More particularly, the present invention relates to a non-aqueous electrode secondary battery having a negative electrode including a specific binder and a non-aqueous electrolyte with a low salt concentration, which exhibits good high-rate discharge characteristics and low-temperature characteristics, and which also ensures high safety.
The present invention also relates to a negative electrode including a specific binder and an active material, of which the surface area contributable to the charging and discharging reaction is sufficient while securing the strength of the electrode.
Specifically, the present invention relates to a non-aqueous electrolyte secondary battery comprising: a positive electrode comprising a compound oxide containing lithium; a negative electrode comprising a carbon material; a separator interposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte comprising a non-aqueous solvent and LiPF6 dissolved therein, wherein the negative electrode contains 0.6 to 1.7 parts by weight of a particulate modified styrene-butadiene rubber and 0.7 to 1.2 parts by weight of a thickening agent per 100 parts by weight of the carbon material where the total amount of the particulate modified styrene-butadiene rubber and the thickening agent is 1.3 to 2.4 parts by weight per 100 parts by weight of the carbon material, and the concentration of LiPF6 in the non-aqueous electrolyte is 0.6 to 1.05 mole/liter.
Herein, the particulate modified styrene-butadiene rubber preferably contains a copolymer comprising an acrylonitrile unit, a styrene unit, and a butadiene unit.
All or a part of the copolymer is preferably in a form of a core-shell type particle.
In a FT-IR absorption spectrum of the copolymer comprising an acrylonitrile unit, a styrene unit, and a butadiene unit, the intensity of the absorption peak attributed to Cxe2x89xa1N stretching vibration in the acrylonitrile unit is preferably 0.1 to 2, and more preferably 0.1 to 0.5 times the intensity of the absorption peak attributed to Cxe2x95x90C stretching vibration in the butadiene unit.
It is preferable that the mean particle size of the particulate modified styrene-butadiene rubber is 0.05 to 0.4 xcexcm.
It is also preferable that the thickening agent is carboxymethyl cellulose.
It is still also preferable that the concentration of LiPF6 in the non-aqueous electrolyte is 0.7 to 0.9 mole/liter.
The positive electrode preferably contains 0.4 to 2 parts by weight of a particulate modified acrylic rubber per 100 parts by weight of the compound oxide containing lithium.
The particulate modified acrylic rubber preferably contains a copolymer comprising a 2-ethylhexylacrylate unit, an acrylic acid unit, and an acrylonitrile unit.
In a FT-IR absorption spectrum of the copolymer comprising a 2-ethylhexylacrylate unit, an acrylic acid unit, and an acrylonitrile unit, the intensity of the absorption peak attributed to Cxe2x95x90O stretching vibration in the 2-ethylhexylacrylate unit and acrylic acid unit is preferably 3 to 50 times the intensity of the absorption peak attributed to Cxe2x89xa1N stretching vibration in the acrylonitrile unit.
The present invention also relates to a negative electrode for a non-aqueous electrolyte secondary battery comprising: a carbon material as an active material; 0.6 to 1.7 parts by weight of the particulate modified styrene-butadiene rubber as a binder per 100 parts by weight of the carbon material; and 0.7 to 1.2 parts by weight of a thickening agent per 100 parts by weight of the carbon material, wherein the total amount of the particulate modified styrene-butadiene rubber and the thickening agent is 1.3 to 2.4 parts by weight per 100 parts by weight of the carbon material.
The present invention still also relates to a negative electrode for a non-aqueous electrolyte secondary battery comprising: a carbon material as an active material; and the particulate modified styrene-butadiene rubber as a binder; wherein the surface area of the carbon material is 300 to 600 m2 per 1 gram of the particulate modified styrene-butadiene rubber.
Note that in the FT-IR absorption spectrum, the intensity of the absorption peak is obtained as the height of the absorption peak from the base line of the spectrum.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.